Imaging ultrasound systems used for medical diagnostic purposes most often comprise an ultrasound transducer having an array of piezoelectric elements. When an electrical voltage is supplied across a piezoelectric element, the piezoelectric element will change its physical dimensions in one or more directions in response to the supplied voltage. In imaging ultrasound systems a short electrical pulse is supplied over the piezoelectric elements, which excites the elements and causes them to oscillate at their ultrasound resonance frequency, which is typically in the megahertz range. The oscillation is a burst with a short duration. When the array of piezoelectric elements is placed in contact with the skin of a human being or an animal, the combined ultrasound burst from the piezoelectric elements will propagate into the body of the human being or the animal. Soft tissue in the body will transmit the ultrasound into the body, and boundaries between organs and other irregularities, eg within an organ, will reflect and scatter a fraction of the ultrasound energy. A fraction of the reflected ultrasound energy will reach the piezoelectric elements of the transducer as an echo and cause the elements to vibrate in response to the received echo, whereby each of the piezoelectric elements will generate an electrical signal corresponding to the received, reflected ultrasound energy. The thus generated electrical signals are analysed and processed to give information about the structures in the body, and the information can be processed to give eg image information.
In order to have good spatial resolution and a good signal-to-noise ratio, the excitation signals to the array of piezoelectric elements are phased or delayed relative to each other, whereby the ultrasound energy from the transducer can be focused to a beam in a desired direction and at a desired distance from the transducer. By proper relative phasing or delaying the excitation signals the beam can be scanned or swept across a plane, whereby a rectangular area or an angular sector in the body is scanned. Scanning or sweeping the scan lines can also be achieved by electronically shifting the transducer aperture of active transducer elements along the length of the transducer.
Echo signals received from a particular point of observation at a certain distance in a certain direction from the transducer will reach the piezoelectric elements of the transducer at different times due to the non-uniform distances from the point of observation to the piezoelectric elements. In order to obtain optimum directivity at each particular depth or distance from the transducer, ie good spatial resolution in the direction transverse to the beam, the electrical signals corresponding to the echo signals received by the elements in the transducer aperture, are therefore delayed dynamically relative to one another as a function of their distance. This results in a receive focusing and is called beam forming. The delay function depends upon the echo distance from the transducer and the physical dimensions of the transducer. As the echo-distance increases during the receive time of the scan line, the delay function must be correspondingly adapted to change the focus of the beam, so that the focus of the receive beam is changed adaptively. This is called dynamic receive focusing, or DRF. The sum of all delayed signals represents the final beam signal. Signals from other directions will not be time correlated and consequently their signals will be attenuated relative to the main beam signal. The obtained degree of beam directivity strongly depends upon the number of signal input channels present in the beam former circuit.
To obtain a constant beam shape or directivity as a function of distance, it is required that the ratio between the receive distance and the physical width of the transducer aperture is constant. This ratio is limited by the maximum number of signals from transducer elements which can be processed by the receive system. For echoes received from objects close to the transducer surface the aperture must be narrow, and to receive at larger distances, the aperture must be correspondingly wider. For each scan line the number of active transducer elements is increased currently to give a constant ration to the receive distance, and at a certain distance the aperture is fully opened, ie to maximum physical width, and from this point on the beam shape will widen and will no longer be constant as a function of distance. The function of opening the aperture, ideally from one element and incrementally up to the maximum number of aperture elements, is called the aperture function. The relation between receive distance and aperture width is called the aperture FIGURE. Due to the aperture function, the DRF system must also provide means of changing the width of the aperture, this because that the aperture opening is also a dynamic function following the distance of receiving and implementing a certain desired aperture FIGURE.
The most common way to obtain DRF in analog beam formers is to have two complete sets of delay circuits. While one delay circuit is being used for receiving and processing ultrasound echoes from a certain depth, the other circuit is being programmed with delay values corresponding to the next, greater depth. Then the roles are reversed, and each circuit thus alternates between signal processing and circuit-programming, and during the time between two alternations the apparatus receives and processes echo signals from a certain depth interval or zone. This method is known as the zone method or "ping-pong" method. Each zone represents some receive time (a few microseconds), and thereby a range of a few millimeters in tissue depth. While the output signal from one zone circuit is processed, the other zone circuit is programmed to obtain focus in the receive zone to follow. This concept needs two complete sets of perfectly matching delay lines and switching components. It also requires a very high performance signal switch for the real time alternation between the two zone-outputs. The concept does however perform in real time, meaning that only one scan line is needed to obtain the complete scan line information. Therefore it does not limit the frame rate of the ultrasound scanner.
With the "ping pong" method a constant focus setting is maintained at minimum for the depth of one depth-zone, ie a few microseconds. This implies that that the opening of the aperture must follow the steps in distance given by the zone width. A consequence of this is deviations in the resulting beam shape, as the aperture FIGURE is not constant within any zone.
A cheaper way of implementing the DRF is to use just one set of delay and switch components. Doing this, settings of the beam former circuit are performed at intervals along the scan line. The output signal from this process contains zones with correct receive information alternating with zones with programming noise. Then another scan line is generated in the same direction. For this second scan line the focus programming is performed time-shifted relative to the first line. The two sets of information are then combined digitally into one complete scan line without programming noise. The quality of such a composite receive focus system can be quite sufficient, but as it make use of at least two scan lines to create one line, and the frame rate of the scanner is reduced by at least 50% relative to a real time system.